Expose descriptor metadata: name, relation, alias, owner, charSetId, subType

[fdcastel]

Summary

Add six new metadata fields to the Descriptor struct, populated from IMessageMetadata during statement preparation. This resolves the // FIXME: more things placeholder and provides ODBC-level metadata access without requiring consumers to use the low-level Firebird API directly.

Changes

Descriptor.h — Six new fields added to Descriptor:

Field	Source (IMessageMetadata)	Description
name	getField()	Column or parameter name
relation	getRelation()	Table/relation name (empty for expressions)
alias	getAlias()	Column alias from the SELECT list
owner	getOwner()	Relation owner (empty for expressions)
charSetId	getCharSet()	Character set ID
subType	getSubType()	BLOB sub-type or numeric sub-type

Statement.cpp — Fields populated in the processMetadata lambda alongside existing descriptor initialization.

test/Statement.cpp — New descriptorMetadataFields test case that:

• Creates a table with integer, varchar, numeric, and text blob columns
• Queries with aliases and verifies name, alias, relation, subType on output descriptors
• Verifies input parameter descriptors have empty names/relations/aliases

Notes

• Non-breaking, additive change — all existing code is unaffected
• Field naming follows maintainer's request from Descriptor Metadata Access: expose field, relation, alias, owner, charSet, and subType #37: name (not field) and charSetId (not charSet)
• Raw IMessageMetadata accessors (getInputMetadata() / getOutputMetadata()) remain available for advanced use cases

Closes #37